1. Field of the Invention
The present invention relates to a disk brake that generates a braking force using an electric motor.
2. Background Art
An example of a background art in the field of the present invention includes a disk brake that generates a braking force using torque of an electric motor. Such a disk brake includes an electric motor in a caliper, and includes a mechanism that converts torque of the electric motor into a linear motion and a braking force holding mechanism. In the disk brake, a piston of the caliper is thrust by energization to the electric motor, and a brake pad is pressed against a disk rotor mounted to wheels (hereinafter referred to as a clamp operation). Thus, a braking force is generated, and this state is held to stop rotation of wheels in parking.
Such a disk brake includes a controller, and controls forward rotation and reverse rotation of the electric motor based on determination according to a switch operation by a driver or a vehicle state to switch a braking state. Usually, the controller observes a current to the electric motor in energization, and when the current reaches a target current value, the controller determines that the piston generates a target thrust force, and stops energization.
However, when a temperature of the brake pad is high, the disk brake is operated with the brake pad being thermally expanded. If a vehicle is left stopped when the target current value is set to a constant value, the brake pad is contracted with reduction in temperature to reduce a thrust force against the disk rotor. There is concern that the thrust force may be reduced to less than a thrust force that allows a parking state to be maintained (hereinafter referred to as a required thrust force).
Thus, in order to ensure durability that allows a large thrust force anticipating a reduction amount to be generated, a caliper is increased in size. Specifically, there is a method of generating a large thrust force at one time so that a required thrust force can be maintained even if a brake pad is contracted, but in that case, strength or the like of a caliper needs to be ensured to generate a larger thrust force, which may disadvantageously increase a size and thus a weight of the caliper.
As a measure against a reduction in thrust force without increasing a size of a caliper, a technique is proposed such that a disk brake includes thrust force detection means, the thrust force detection means is used to monitor a reduction in thrust force, and a clamp operation is performed every time the thrust force reaches a required thrust force (for example, see JP Patent Publication (Kokai) No. 2002-225701).
Also, a technique is proposed such that thrust force detection means detects a thrust force after a clamp operation of a disk brake, a thrust force reduction amount after cooling of a brake pad is estimated from a change in thrust force with time, and it is determined whether another clamp operation is required or not based on the estimation (for example, see JP Patent Publication (Kokai) No. 2006-232263).
However, as in the above-described disk brake, separately providing a force sensor in the disk brake increases complexity or size of a structure of the disk brake, and it is difficult to apply such a technique to the disk brake desired to have a small and simple structure.